This invention relates to fabrication technologies for semiconductor devices and, more particularly, to a surface contamination analyzer for semiconductor wafers, a method used therein and a process for fabricating a semiconductor device.
Semiconductor devices have been enhanced in integration density, and, accordingly, miniature circuit components are integrated on the small semiconductor chip. If the surface of a semiconductor wafer is contaminated with trace elements, the miniature circuit components are much liable to be damaged, and the production yield is lowered. Research and development efforts are being made on 1 giga-bit DRAM (Dynamic Random Access Memory). The contamination due to the trace elements is serious in the process for fabricating the DRAMs. Of course, miniaturization is the key technologies in the development. A cleaning technology is also important for the highly reliable ultra large scale integration devices. In fact, the requirement for an ultra clean surface is getting severe and severe.
Organic compounds in the clean room and plasticizer in the wafer cassette are presently seemed to be origins of the contaminants. The organic contaminants are adhered to the surfaces of the semiconductor wafers in the form of molecules and/or cluster, and are causative of reduction in production yield. A particle of organic compound is assumed to be adhered to the surface of a semiconductor wafer. The organic compound particle is an obstacle in the removal of natural oxide. Even though the manufacturer exposes the surface of the semiconductor wafer through the removal of the natural oxide before the deposition of metal, the organic compound particle does not allow the etchant to remove the natural oxide therebeneath. This means that a part of the metal layer is held in contact with the residual natural oxide on the surface of the semiconductor wafer. Although the resistance in the direct contact between the metal layer and the semiconductor wafer is fallen within a target range, the contact resistance is locally increased, and uniform contact resistance is not achieved.
Organic contaminant particles are assumed to be adhered to a surface of semiconductor wafer. Dopant impurity may be ion implanted into the surface of the semiconductor wafer/semiconductor layer. The organic contaminant particles are also obstacle against the ion-implantation, and do not allow the impurity region to have a target impurity profile. If the impurity region is used for a channel region or diode, the field effect transistor or diode does not exhibit designed characteristics. Especially, the channel region forms a part of flash memory. Electrons or holes are injected and evacuated through the gate oxide layer between the channel region and the floating electrode. The residual organic contaminant particles accelerate the aged deterioration, and, accordingly, reduce the duration of life as reported by Toshiyuki Iwamoto in xe2x80x9cResearch for Highly Reliable Extremely Thin Oxide Layerxe2x80x9d, dissertation for a doctor degree, Tohoku University, March, 1998.
Semiconductor wafers are concurrently conveyed from an apparatus to another as a lot. Since the time period consumed in an apparatus is different from the time period consumed in another apparatus, the semiconductor wafers are to wait until the previous lot is unloaded from the apparatus, and are exposed to the atmosphere in the clean room. While the semiconductor wafers are being exposed to the atmosphere, the molecular contaminants or contaminant clusters are unavoidably adhered to the surfaces of the semiconductor wafers. The waiting time is different between the lots, and, accordingly, the semiconductor wafers are different in degree of contamination from one another. The contaminants are influential in the electric properties of the semiconductor wafers. Thus, accurate evaluation technologies are required for the ultra large-scale integration devices.
A wide variety of material is used in the processes for fabricating the ultra large-scale integration devices. Silicon dioxide, i.e., SiO2 is popular to the semiconductor device manufacturers. Other kinds of insulating material such as, for example, SiOF, HSQ, SiN and Ta2O5 are employed in the ultra large-scale integration devices. Semiconductor wafers are cleaned in certain cleaning solution such as water solution containing sulfuric acid and hydrogen peroxide or water solution containing ammonia and hydrogen peroxide. The contaminants are removed through chemical reactions with these kinds of cleaning solution. However, new cleaning technologies are required for the new kinds of material. Physical phenomena are employed in the cleaning technologies for those kinds of material. The contaminants are, by way of example, removed from semiconductor wafers by using mega-sonic or ultra sonic. These cleaning technologies are effective against contaminants on the surfaces of the semiconductor wafers. However, the cleaning solution and/or the physical energy hardly reaches the contaminants in deep valleys or trenches and micro-holes formed in the three-dimensional structure. If the contaminants are left on the bottom surfaces of the trenches and micro-holes, the residual contaminants vary the electric properties, and the circuit components formed in the trenches and micro-holes do not exhibit designed characteristics. The defective circuit components are causative of reduction in production yield. Even though the products pass the inspections, malfunction is liable to take place in the products, and, accordingly, those products are less reliable.
In order to cope with the contamination, it is necessary to evaluate the surface contamination without any damage to the complicated surface configuration for optimizing the cleanings. A total reflection fluorescent x-ray spectroscopy has been developed for the surface evaluation. The total reflection fluorescent x-ray spectroscopy is usually abbreviated as xe2x80x9cTXRFxe2x80x9d. Monochromatic x-ray is radiated to the surface of a semiconductor wafer at a small incident angle. The semiconductor wafer and contaminants generate fluorescent x-ray to a semiconductor detecting unit, and the contaminants are specified on the basis of the fluorescent x-ray incident onto the semiconductor detecting unit. Metal contaminants are well detected through the total reflection fluorescent x-ray spectroscopy. However, contaminants essentially composed of carbon-containing/nitrogen-containing molecules are hardly detected through the total reflection fluorescent x-ray spectroscopy. Moreover, the mono-chromatic x-ray does not reach the bottom surfaces of extremely deep trenches, because the incident angle is small, and the contaminants on the bottom surfaces are not detectable.
A surface contamination analyzer with an optical detector is disclosed in Japanese Patent Application laid-open No. 7-221148. Infrared light is radiated from a light source to the surface of a semiconductor wafer, and the reflected infrared light is incident on the surface at a predetermined angle. The infrared light is reflected on the surface, again, and the reflection is analyzed through a spectral analysis. The contaminants essentially composed of carbon-containing/nitrogen-containing molecules are detected without serious damage to the semiconductor wafer.
Another prior art surface contamination analyzer is disclosed in Japanese Patent Application laid-open No. 9-243535. Carrier gas flows over the surface of a semiconductor wafer, and laser light is radiated onto a certain area of the surface. The contaminants are vaporized in the radiation of the laser light, and the gaseous contaminant is ionized together with the carrier gas. The ionized gas is analyzed through a mass spectrometry, and the contaminants are specified on the basis of the analysis result. Even if the contaminants are on the bottom surfaces of the extremely deep trenches, the laser light reaches the contaminants so as to vaporize them. Thus, the contaminants in the extremely deep trenches are analyzable through the second prior art surface contamination analysis.
A problem inherent is encountered in the first prior art surface contamination analysis technology in a small signal-to-noise ratio of the reflection from the trenches/holes with large aspect ratios. This is because of the fact that the first prior art surface contamination analyzer is to keep the incident light at the predetermined angle. In other words, the limit is set on the incident angle in the surface contamination analysis so that the reflected infrared light does not reach the bottoms of the trenches/holes with the large aspect ratios.
On the other hand, a problem inherent in the second prior art surface contamination analyzer is a small signal-to-noise ratio of the vapor generated from an extremely narrow area. When the vapor is generated from a relatively wide area, a large amount of contaminants is vaporized, and the mass spectrometry is surely reliable. However, the amount of contaminants vaporized from an extremely narrow area is too little to analyze it through the mass spectrometry. Thus, the second prior art surface contamination analyzer is not available for a detailed report.
If the analysis on the surface contamination is inaccurate, it is difficult to optimize the cleaning against the contamination, and the residual contaminants are carried from an apparatus to another through the contaminated semiconductor wafer. This results in reduction in production yield and poor reliability of semiconductor devices. The surface contamination analysis is an important step in the process for fabricating a semiconductor device.
It is therefore an important object of the present invention to provide a surface contaminant analyzer, which is available for a detailed report on a surface contamination with carbon, nitrogen-containing molecules or cluster regardless of the surface configuration of a specimen.
It is also an important object of the present invention to provide an analyzing method used in the surface contamination analyzer.
It is another important object of the present invention to provide a process for fabricating a semiconductor device through which the production yield and reliability of semiconductor device are enhanced.
In accordance with one aspect of the present invention, there is provided a surface contamination analyzer comprising an electron beam radiating unit including an electron gun for radiating an electron beam along a certain path, and a current measuring unit including a wall defining a chamber into which the certain path extends, a movable stage mounting a sample and moved so as to align a target region with the certain path and a current measuring equipment electrically connected between the sample and a constant voltage source for measuring the amount of current flowing out from the target region under the radiation of the electron beam onto the target region.
In accordance with another aspect of the present invention, there is provided a method for investigating a degree of contamination on a target region of a contaminated sample comprising the steps of a) aligning the target region of the contaminated sample with a path of an electron beam, b) measuring the amount of current flowing out from the target region under radiation of the electron beam, c) comparing the amount of current with the amount of reference current flowing out from a region of a reference sample corresponding to the contaminated sample for determining a difference between the amount of current measured and the amount of reference current, and d) determining the degree of contamination on the target region on the basis of the difference.
In accordance with yet another aspect of the present invention, there is provided a process for fabricating a semiconductor device comprising the steps of a) treating the semiconductor water in an atmosphere potentially having an origin of contamination, b) investigating a degree of contamination on the semiconductor wafer through sub-steps of b-1) aligning a target region of the semiconductor wafer with a path of an electron beam, b-2) measuring the amount of current flowing out from the target region under radiation of the electron beam, b-3) comparing the amount of current with the amount of reference current flowing out from a region of a reference wafer corresponding to the semiconductor wafer for determining a difference between the amount of current measured and the amount of reference current and b-4) determining the degree of contamination on the semiconductor wafer on the basis of the difference, c) evaluating the degree of contamination to see whether or not a cleaning is required for the semiconductor wafer, d) decontaminating the semiconductor wafer when the answer at step c) is given affirmative and e) treating the semiconductor wafer in another next atmosphere.